Electrode for nonaqueous electrolyte secondary battery, method for producing same, and nonaqueous electrolyte secondary battery comprising such electrode for nonaqueous electrolyte secondary battery

ABSTRACT

An electrode for a non-aqueous electrolyte secondary battery  6  according to the present invention includes: a current collector  3 ; a first active material layer  2  formed on the current collector  3 ; and a second active material layer  5  provided on the first active material layer  2 , the second active material layer  5  including a plurality of active material particles  4 . The plurality of active material particles  4  is mainly of a chemical composition represented as SiO x (0≦x&lt;1.2). The first active material layer  2  is mainly of a chemical composition represented as SiO y (1.0≦y&lt;2.0, y&gt;x). The area in which the first active material layer  2  is in contact with the plurality of active material particles  4  is smaller than the area in which the current collector  3  is in contact with the first active material layer  2.

TECHNICAL FIELD

The present invention relates to an electrode for a non-aqueouselectrolyte secondary battery and a method of producing the same, and anon-aqueous electrolyte secondary battery having an electrode for anon-aqueous electrolyte secondary battery.

BACKGROUND ART

In recent years, as mobile devices improve in performance and acquiremore functions, secondary batteries serving as their power supplies arerequired to have higher and higher capacity. Lithium-ion secondarybatteries are drawing attention as secondary batteries that can satisfythis need.

In order to achieve a high capacity in a lithium-ion secondary battery,use of silicon (Si), germanium (Ge), tin (Sn), or the like as anelectrode active material has been proposed. An electrode for alithium-ion secondary battery (which hereinafter may also be simplyreferred to as an “electrode”) for which such an electrode activematerial is used is formed by applying on a current collector a slurrywhich contains an electrode active material, a binder, and the like, forexample (called a “painted electrode”).

However, such electrode active materials undergo large changes in volumewhen occluding or releasing lithium ions, thus resulting in the problemof being pulverized through expansions and contractions that accompanyrepetitive charge and discharge. As an electrode active material becomesfine matter by being pulverized, a deterioration in the chargecollecting ability of the electrode will occur. This will also increasethe area of contact between the electrode active material and theelectrolytic solution, thus promoting a decomposition reaction of theelectrolytic solution by the electrode active material. This makes itimpossible to obtain sufficient charge-discharge cycle characteristics.

Patent Document 1 and Patent Document 2 disclose, instead of aconventional painted electrode, forming an electrode active materiallayer on a current collector by using vapor-phase technique,liquid-phase technique, sintering technique, or the like. With anelectrode which has been formed in this manner, as compared to aconventional painted electrode, the electrode active material's tendencyto become fine matter through pulverization can be suppressed, and theadhesion between the current collector and the electrode active materiallayer can be enhanced, whereby deterioration in charge collectingability can be prevented. Thus, it is expected that improvements inelectrode capacity and cycle life beyond conventional levels areexpected. Furthermore, whereas a conventional painted electrode wouldinclude an electrically conductive material, a binder, voids, etc., themethods of forming electrode active material layers disclosed in PatentDocuments 1 and 2 can reduce or eliminate their amounts within theelectrode, which permits an essential enhancement of the capacity of theelectrode.

However, even with the aforementioned electrode, expansions andcontractions of the electrode active material due to charge anddischarge may cause problems such as peeling of the electrode activematerial layer from the current collector, wrinkles occurring on thecurrent collector, etc., thus making it difficult to obtain sufficientcycle characteristics.

In view of this, it has been proposed to form an intermediate layerbetween the current collector and the electrode active material layer,thereby improving the adhesion between the current collector and theelectrode active material layer and preventing peeling of the electrodeactive material layer from the current collector. Patent Document 3discloses a construction in which a current collector composed of ametal or alloy having a high mechanical strength is used and anintermediate layer that can be alloyed with the electrode activematerial, e.g., copper (Cu), is provided between the current collectorand the electrode active material layer. Patent Document 4 discloses aconstruction in which an intermediate layer containing molybdenum (Mo)or tungsten (W) is provided between the current collector and theelectrode active material layer. Patent Document 5 discloses aconstruction in which an intermediate layer containing nickel (Ni) andtitanium (Ti) is provided.

Patent Document 6 discloses performing an oxidation treatment for thesurface of the current collector, and forming on the resultant oxidefilm an electrode active material layer which contains at least one ofSi and Ge.

On the other hand, Patent Document 7 takes note of the fact that, in thecase where silicon oxide is used as an electrode active material, theexpansion coefficient of the electrode active material associated withcharge and discharge varies depending on the oxygen content in thesilicon oxide, thus proposing to increase the oxygen concentration inthe electrode active material layer near the current collector so as tobe higher than the average oxygen concentration in the electrode activematerial layer. In accordance with the construction of Patent Document7, the expansion of the electrode active material layer near the currentcollector is reduced, thus suppressing deformation of the electrode dueto expansion/contraction of the electrode active material layer.

[Patent Document 1] Japanese Laid-Open Patent Publication No. 11-339777

[Patent Document 2] Japanese Laid-Open Patent Publication No. 11-135115

[Patent Document 3] Japanese Laid-Open Patent Publication No.2002-083594

[Patent Document 4] Japanese Laid-Open Patent Publication No.2002-373644

[Patent Document 5] Japanese Laid-Open Patent Publication No.2005-141991

[Patent Document 6] Japanese Laid-Open Patent Publication No.2003-217576

[Patent Document 7] Japanese Laid-Open Patent Publication No.2006-107912

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in any of the constructions disclosed in Patent Documents 3, 4,and 5, an intermediate layer is provided between the current collectorsubstrate material (Cu) and the electrode active material (Si), theintermediate layer containing a material other than the materials of thecurrent collector and the electrode active material. Therefore,differences in expansion coefficient between the current collector, theintermediate layer, and the electrode active material layer may causepeeling at the interfaces between them. In the construction disclosed inPatent Document 6, too, peeling may occur at the interface between theelectrode active material layer and the current collector due to adifference in expansion coefficient between the electrode activematerial layer and the current collector. Thus, peeling of the electrodeactive material layer cannot be adequately suppressed with theconstructions of Patent Documents 3 to 6, and it is difficult toeffectively enhance the cycle characteristics.

Moreover, with the construction proposed in Patent Document 7,particularly in the case where there is a thick electrode activematerial layer containing silicon oxide (e.g., over 10 μm), theexpansion amounts of silicon oxide due to charge and discharge will havevery large absolute values. Therefore, just increasing the oxygenconcentration in the electrode active material layer near the currentcollector cannot completely suppress electrode deformation.

The present invention has been made in view of the aforementionedproblems, and an objective thereof is to alleviate the stress acting ona current collector due to expansions and contractions associated withcharge and discharge of an electrode active material, thus improving thecycle characteristics of a non-aqueous electrolyte secondary battery.

Means for Solving the Problems

In order to solve the above problems, an electrode for a non-aqueouselectrolyte secondary battery according to the present inventionincludes: a current collector; a first active material layer formed onthe current collector; and a second active material layer provided onthe first active material layer, the second active material layerincluding a plurality of active material particles, wherein theplurality of active material particles are mainly of a chemicalcomposition represented as SiO_(x)(0≦x<1.2); the first active materiallayer is mainly of a chemical composition represented asSiO_(y)(1.0≦y<2.0, y>x); and an area in which the first active materiallayer is in contact with the plurality of active material particles issmaller than an area in which the current collector is in contact withthe first active material layer.

In accordance with the electrode for a non-aqueous electrolyte secondarybattery of the present invention, a first active material layer isformed between the active material particles and the current collector,the first active material layer having a higher oxygen concentration,i.e., a smaller coefficient of volumetric expansion, than that of theactive material particles. Therefore, while maintaining the lithiumocclusion ability of the active material particles, the stress acting onthe current collector due to expansions and contractions of the activematerial particle can be alleviated.

Since the area in which the current collector is in contact with thefirst active material layer is made larger than the area in which thefirst active material layer is in contact with the plurality of activematerial particles, the adhesion between the active material particlesand the current collector can be greatly improved over the case whereonly the active material particles are formed on the current collector.Moreover, even if the active material particles expand by occludinglithium ions, spaces for such expansions can be secured between theactive material particles, whereby the stress between the activematerial particles can be alleviated.

Thus, deformation of the electrode as well as peeling of the firstactive material layer and the active material particles from the currentcollector surface through repetitive charge and discharge can besuppressed, whereby a non-aqueous electrolyte secondary battery havingexcellent charge-discharge cycle characteristics can be provided.

EFFECTS OF THE INVENTION

With the electrode for a non-aqueous electrolyte secondary batteryaccording to the present invention, the stress acting on a currentcollector due to expansions and contractions of active materialparticles associated with charge and discharge can be alleviated, andelectrode deformation due to concentration of stress on the currentcollector can be suppressed.

Therefore, by employing the electrode for a non-aqueous electrolytesecondary battery according to the present invention for anelectrochemical device such as a non-aqueous electrolyte secondarybattery (e.g., a lithium-ion secondary battery) or an electrochemicalcapacitor, the charge-discharge cycle characteristics of theelectrochemical device can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A schematic cross-sectional view of an electrode according to anembodiment of the present invention.

FIG. 2 A diagram for explaining effects of an embodiment of the presentinvention, being a schematic cross-sectional view showing a constructionin which active material particles are formed directly on the currentcollector surface.

FIGS. 3 (a) and (b) are a cross-sectional view and a plan view,respectively, illustrating another current collector constructionaccording to the present invention.

FIG. 4 A schematic cross-sectional view illustrating another electrodeconstruction according to the present invention.

FIG. 5 A schematic diagram for explaining an exemplary method ofproducing an electrode according to a first embodiment of the presentinvention.

FIG. 6 A schematic diagram for explaining an exemplary method ofproducing an electrode according to a first embodiment of the presentinvention.

FIG. 7 A schematic cross-sectional view illustrating a construction of alithium-ion secondary battery according to the present invention.

FIG. 8 A schematic diagram for explaining an exemplary method ofproducing an electrode according to a second embodiment of the presentinvention.

FIG. 9 A graph showing charge-discharge cycle characteristics of coinbatteries in which electrodes for a lithium-ion secondary batteryaccording to Examples of the present invention and Comparative Exampleare used.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   2 first active material layer    -   3, 73 current collector    -   4, 74 active material particle    -   5 second active material layer    -   6 electrode    -   10 vacuum chamber    -   11 supply roll    -   12 take-up roll    -   13 pulley    -   14, 14A, 14B substrate cooling roll    -   15, 16 masking plate    -   17, 18 oxygen nozzle    -   19 film thickness monitor    -   20, 21 oxygen flow rate controller    -   22 Si raw material (evaporation source)    -   23 evaporation pot    -   24 electron beam radiation system    -   25 vacuum pump    -   26 first active material layer formation zone    -   27 active material particle-forming zone    -   30 current collector on which a first active material layer is        formed    -   100, 200, 300 vacuum deposition apparatus

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Hereinafter, with reference to the drawings, embodiments of theelectrode for a lithium-ion secondary battery according to the presentinvention will be described.

First, FIG. 1 is referred to. FIG. 1 is a schematic cross-sectional viewof an electrode for a lithium-ion secondary battery (hereinafter simplyreferred to as an “electrode”) according to the present embodiment.

An electrode 6 includes a current collector 3, a first active materiallayer 2 formed on the surface of the current collector 3, and a secondactive material layer 5 provided on an upper face of the first activematerial layer 2. The second active material layer 5 is composed of aplurality of active material particles 4. The first active materiallayer 2 is formed so as to cover the surface of the current collector 3,and is mainly of a chemical composition represented as SiO_(y)(1.0≦y<2.0). The plurality of active material particles 4 are mainly ofa chemical composition represented as SiO_(x) (0≦x<1.2, y>x). Moreover,the area in which the plurality of active material particles 4 are incontact with the first active material layer 2 is smaller than the areain which the current collector 3 is in contact with the first activematerial layer 2.

In the present embodiment, a rugged foil having a large surfaceroughness Ra (Ra: e.g. no less than 0.3 μm and no more than 5.0 μm) isused as the current collector 3. A plurality of bumps 3 a are present onthe surface of the current collector 3, such that each active materialparticle 4 is formed on a bump 3 a via the first active material layer2.

Note that FIG. 1 shows the electrode 6 during discharging, where theplurality of active material particles 4 are located on the surface ofthe current collector 3 with intervals therebetween. During charging,these active material particles 4 will expand so that the intervalsbetween adjoining active material particles 4 are reduced, whereby theadjoining active material particles 4 may be partially in contact witheach other.

In the present specification, the aforementioned x, y in the chemicalcompositions of the active material particles 4 and the first activematerial layer 2 represent average values of molar ratios of oxygenamounts, with respect to silicon amounts, in the active materialparticles 4 and the first active material layer 2, respectively (whichhereinafter may also be simply referred to as “mole fractions ofoxygen”). Note that the chemical compositions of the active materialparticles 4 and the first active material layer 2 are meant ascompositions excluding any lithium that may have been added to oroccluded by the active material particles 4 and the first activematerial layer 2. When saying that the active material particles 4 andthe first active material layer 2 are mainly of the aforementionedchemical compositions, “mainly of” indicates that the active materialparticles 4 and the first active material layer 2 substantially have theaforementioned chemical compositions, and may contain impurities such asFe, Al, Ca, Mn, Ti, and C.

In the aforementioned chemical composition, x and y are obtained by,calculating molar ratios of oxygen amounts with respect to Si amounts,based on an Si amount determined with an ICP Atomic EmissionSpectrometer and an oxygen amount determined by combustion analysistechnique, for example. They may alternatively be obtained by usingfundamental parameter technique in an x-ray fluorescence spectrometryusing the O-Kα line.

In the above construction, the first active material layer 2 having ahigher oxygen concentration than that of the active material particles 4is formed between the current collector 3 and the second active materiallayer 5 including the active material particles 4. Therefore, the stressacting on the surface of the current collector 3 due toexpansion/contraction of the active material particles 4 can bealleviated. Hereinafter, with reference to the drawings, the reasonsthereof will be described in detail.

Generally speaking, in an electrode active material (SiO_(z), 0≦z<2)containing silicon, as its oxygen concentration becomes lower (i.e., theaforementioned z becomes smaller), the lithium ions have a higherocclusion ability and thus a higher charge-discharge capacity isobtained, but the coefficient of volumetric expansion due to chargingincreases. On the other hand, as the oxygen concentration in theelectrode active material (i.e., the aforementioned z increases), thecoefficient of volumetric expansion will be reduced but thecharge-discharge capacity will become lower. To specifically describesuch coefficients of volumetric expansion rates of the electrode activematerial, when the electrode active material is silicon (z=0), theelectrode active material will undergo a volumetric expansion by about400% due to charging. When the amount of oxygen atoms with respect tosilicon atoms in the electrode active material is 30% (z=0.3), theelectrode active material will undergo a volumetric expansion by about350% due to charging. Similarly, when the amount of oxygen atoms is 60%(z=0.6), a volumetric expansion by about 280% occurs; and when theamount of oxygen atoms is 100% (z=1.0), a volumetric expansion by about200% occurs.

Now, a construction for forming a plurality of active material particlesdirectly on the surface of the current collector by using an electrodeactive material (SiO_(z), 0≦z<2) will be discussed. As shown in FIG. 2,in a construction where active material particles 74 are formed directlyon the surface of the current collector 73, each active materialparticle 74 tries to isotropically expand upon charging. At this time,assuming that the plurality of active material particles 74 and thevoids therebetween constitute a single film 75, each active materialparticle 74 is capable of isotropic expansion when the volume proportionoccupied by the active material particles 74 in the film 75 (hereinafterreferred to as “film density”) is sufficiently low. However, since thecoefficient of volumetric expansion of the active material particles 74is very high as described earlier, as the film density increases, itbecomes more difficult to secure spaces for the active materialparticles 74 to expand along in-plane directions of the film. As aresult, the active material particles 74 cannot undergo isotropicexpansion, but expands mainly along a film thickness direction 78.Therefore, with such a construction, depending on the composition(oxygen concentration) and film density of the electrode activematerial, the thickness of the active material particles 74, etc., thestress caused by expansion of the active material particles 74 mayconcentrate near the interfaces between the current collector 73 and theactive material particles 74, thus possibly causing peeling of theactive material particles 74 or plate electrode deformation near thecurrent collector 73.

On the other hand, according to the present embodiment, as shown in FIG.1, a first active material layer (SiO_(y), y>x) 2 is provided betweenthe current collector 3 and the active material particles (SiO_(x)) 4,the first active material layer 2 having a higher oxygen concentration(or molar ratio of the oxygen amount with respect to the siliconamount), i.e., a smaller coefficient of volumetric expansion, than thatof the active material particles 4. As a result, while securing theocclusion ability of lithium ions of the active material particles 4,expansion of the active material particles 4 near the first activematerial layer 2 can be suppressed, whereby stress acting on the currentcollector 3 can be alleviated. Therefore, plate electrode deformationdue to concentration of stress on the current collector 3 and peeling ofthe active material particles 4 and the first active material layer 2can be suppressed. Thus, it is possible to enhance the charge-dischargecycle characteristics while ensuring a high charge-discharge capacity.

The first active material layer 2 according to the present embodiment isformed so as to cover the surface of the current collector 3, and itsthickness t2 is generally uniform across the surface of the currentcollector 4. Such a first active material layer 2 can be obtained byvacuum deposition technique, for example. Note that the first activematerial layer 2 only needs to be formed between the current collector 3and the active material particles 4 so as to prevent direct contactbetween the current collector 3 and the active material particles 4, anddoes not need to cover the entire surface of the current collector 3. Asnecessary, the first active material layer 2 may be partially removed inorder to provide a lead for charge collection or the like, for example.

Next, the chemical compositions of the active material particles 4 andthe first active material layer 2 according to the present embodimentwill be discussed.

The value of x in the chemical composition (SiO_(x)) of the activematerial particles 4 is equal to or greater than 0 and less than 1.2.When x is 1.2 or more, it becomes necessary to form thicker activematerial particles 4 for securement of capacity, in which case formationof the active material particles 4 may cause problems such as warping ofthe current collector 3. Preferably, x is 0.7 or less, whereby a highercapacity can be obtained while reducing the thickness of the activematerial particles 4. On the other hand, x of 0.3 or more is preferablebecause volumetric expansion of the active material particles 4 issuppressed and the stress acting on the current collector 3 is furtherreduced.

The value of y in the chemical composition (SiO_(y)) of the first activematerial layer 2 is equal to or greater than 1.0 and less than 2.0. For,electrical conductivity of the first active material layer 2 can beensured when y is less than 2.0, and the coefficient of volumetricexpansion of the first active material layer 2 can be sufficientlylowered when y is 1.0 or more. Preferably, y is 1.6 or less, whereby thedifference in expansion coefficient between the first active materiallayer 2 and the active material particles 4 can be reduced, so that theadhesion between the first active material layer 2 and the activematerial particles 4 can be further enhanced.

Therefore, by controlling x to no less than 0.3 and no more than 0.7(0.3≦x≦0.7) and y to be no less than 1.0 and no more than 1.6(1.0≦y≦1.6), cycle deteriorations can be made very small.

The oxygen concentration profiles of the first active material layer 2and the active material particles 4 may be generally uniform along thethickness direction, or vary along the thickness direction. Even in thecase where they vary along the thickness direction, it suffices if theaverage values of molar ratios (mole fractions of oxygen) of oxygenamounts with respect to silicon amounts in the first active materiallayer 2 and the active material particles 4 satisfy the aforementionedranges of x and y, respectively. For example, the oxygen concentrationin the first active material layer 2 may decrease away from the currentcollector 3 and toward the active material particles 4, whereby adhesionbetween the first active material layer 2 and the active materialparticles 4 can be further enhanced while alleviating concentration ofstress on the current collector 3. Similarly, the oxygen concentrationin each active material particle 4 may be higher near the first activematerial layer 2 and decrease toward the upper face. As a result,expansion of the portion of the active material particle 4 that is nearthe first active material layer 2 can be further decreased, so thatdeterioration in the cycle characteristics due to expansion of theactive material particles 4 can be effectively suppressed.

Preferably, the oxygen concentrations in the first active material layer2 and the active material particles 4 along the thickness directiondecrease from the interior of the first active material layer 2 towardthe interior of the active material particles 4. In other words, it ispreferable that the chemical composition (mole fraction of oxygen) of anelectrode active material which consists of the first active materiallayer 2 and the active material particles 4 gradually changes at thebonding site between the first active material layer 2 and each activematerial particle 4. With such a construction, no interface is createdat the bonding site between the first active material layer 2 and eachactive material particle 4. Therefore, peeling of the active materialparticles 4 from the first active material layer 2 and pulverization ofthe active material particles 4 become less likely to occur, and thecycle characteristics can be further enhanced.

From the standpoint of electrical conductivity, energy density,expansion coefficient, and the like, it is preferable that the thicknesst2 of the first active material layer 2 is greater than 2 nm and lessthan 100 nm. When the thickness t2 of the first active material layer 2is greater than 2 nm, plate electrode deformation near the surface ofthe current collector 3 can be suppressed with an increased certainty.When the thickness t2 is less than 100 nm, and more preferably less than50 nm, decrease in capacity due to the first active material layer 2 canbe reduced, and a sufficient battery energy can be ensured.

The thickness t5 of the second active material layer 5 is equal to thethickness of the active material particles 4 in the present embodiment,and is preferably no less than 0.2 μm and no more than 50 μm from thestandpoint of energy density, high-rate characteristics, productivity,etc., of the battery. When the thickness t5 of the second activematerial layer 5 is no less than 0.2 μm, and more preferably no lessthan 5 μm, a higher battery energy can be obtained. When the thicknesst5 is no more than 50 μm, and more preferably no more than 30 μm, cracksoccurring during formation of the second active material layer 5 can bereduced, whereby the reliability of the electrode 6 can be enhanced.

Note that the oxygen concentrations (mole fractions of oxygen) in thefirst active material layer 2 and the active material particles 4 alongthe thickness direction can be measured by various methods. For example,they can be determined by using fundamental parameter technique in X-rayPhotoelectron Spectroscopy or Electron Spectroscopy for ChemicalAnalysis (XPS, ESCA) or in x-ray fluorescence spectrometry using theO-Kα line. Moreover, in order to measure the oxygen concentration in thefirst active material layer 2 along the thickness direction, Ar etchingmay be performed and the oxygen concentration at a desired thickness maybe measured (e.g., a Marcus radio frequency glow discharge spectrometermanufactured by HORIBA), or, a silicon oxide coating for measurementpurposes that has the intended thickness for measurement may beseparately formed, and an oxygen concentration in its surface layer maybe measured. Alternatively, a sample may be consolidated in resin, whichis then polished to obtain a polished cross section, and the oxygenconcentration and thickness in this polished cross section may bemeasured through EPMA analysis (Electron Probe Micro Analyzer) orwavelength dispersive x-ray microanalysis (Wavelength Dispersive X-raySpectroscopy: WDS).

In the present embodiment, a ratio s1/s2 of an area (s1) in which thefirst active material layer 2 is in contact with the active materialparticles 4 to the area (s2) of the first active material layer 2(hereinafter simply referred to as “coverage S”) is preferably no lessthan 20% and no more than 70%. In the case where the active materialparticles 4 are formed in an oxygen atmosphere by using vacuumdeposition technique, the coverage S can be controlled based on thesurface roughness Ra of the current collector 3 and the incident angleof the silicon (Si) vapor with respect to the current collector 3. Asthe surface roughness Ra increases, shades are more likely to occur inresponse to the incoming particles to be deposited, thus resulting in asmaller coverage S. As the incident angle increases, shades become morelikely to occur and the coverage S tends to become smaller. When thecoverage S is 20% or more, a higher battery energy can be obtainedwithout increasing the thickness t5 of the second active material layer5. When the coverage S is 70% or less, sufficient spaces for expansioncan be secured between the active material particles 4.

The coverage S can be measured by observing a polished cross section ofthe electrode 6 by using an SEM (Scanning Electron Microscope). In thepresent embodiment, the first active material layer 2 covers generallythe entire surface of the current collector 3, and the surface of thefirst active material layer 2 has bumps/dents corresponding to thesurface roughness Ra of the current collector 3; therefore, the coverageS is measured by the following method.

First, in a polished cross section under SEM observation, per length of100 μm, a length A over the surface bumps/dents of the first activematerial layer 2 and a length B of portions of the surface bumps/dentsof the first active material layer 2 that are bonded to the activematerial particles 4 are determined, and B/A is calculated. In thismanner, B/A is calculated at four places in the polished cross section,and an average value thereof (i.e., an average value over a length of400 μm) is defined as the coverage S. Note that, in the case where thesurface of the first active material layer 2 has a shape correspondingto the surface configuration of the current collector 3, as in thepresent embodiment, the length over the surface bumps/dents of thecurrent collector 3 may be adopted as the length A, instead of thelength over the surface bumps/dents of the first active material layer2, and the length of the portions of the surface bumps/dents of thecurrent collector 3 that are bonded to the active material particles 4may be adopted as the length B, for use in the determination of thecoverage S.

The active material particles 4 in the present embodiment are not incontact with one another, but are independent. In this manner, spacesfor accommodating the expansion of the active material particles 4during lithium occlusion can be secured between the active materialparticles 4, with an increased certainty. Note that the construction ofthe active material particles 4 is not limited to the above. It sufficesso long as the second active material layer 5 has sufficient voidsbetween the active material particles 4 during discharging, andadjoining active material particles 4 may be partially in contact withone another.

Furthermore, in the present embodiment, the active material particles 4have a growth direction S which is tilted with respect to the normaldirection D of the surface of the current collector 3. By constructing alithium-ion secondary battery with such an electrode, the area ofportions of a positive-electrode active material layer that oppose thefirst active material layer 2 of the current collector 3 can be reduced.In other words, it is possible to increase the area of the portions ofthe positive-electrode active material layer that oppose the activematerial particles 4, which have a higher capacity than that of thefirst active material layer 2, whereby the charge-discharge efficiencycan be enhanced.

Preferably, the angle between the growth direction S of the activematerial particles 4 and the normal direction D of the current collector3 is 10° or more. When this angle is 10° or more, the area of theportions of the positive-electrode active material layer that oppose theactive material particles 4 can be sufficiently increased, whereby thecharge-discharge efficiency can be enhanced with an increased certainty.On the other hand, the aforementioned angle is less than 90°, but ispreferably less than 80° since it becomes more difficult to form theactive material particles 4 as it becomes closer to 90°.

Such active material particles 4 can be formed by, for example, allowingsilicon and oxygen to be vapor-deposited on the surface of the currentcollector 3 from a direction which is tilted with respect to the normaldirection of the current collector 3 (oblique vapor deposition). Forexample, by performing the aforementioned vapor deposition from an angle(tilting angle) of no less than 60° and no more than 75° with respect tothe normal direction of the current collector 3, the active materialparticles 4 can be grown in a direction (growth direction) which is noless than 30° and no more than 50° with respect to the normal directionof the current collector 3.

There are no particular limitations as to the current collector 3 in thepresent embodiment, but a metal foil such as copper or nickel can beused, for example. The thickness of the foil is preferably 30 μm orless, and more preferably 10 μm. As a result, strength of the electrode6 and volumetric efficiency of the battery can be ensured. On the otherhand, from the standpoint of handling ease, the foil thickness ispreferably 4 μm or more, and more preferably 5 μm or more.

The surface of the current collector 3 may be smooth, but for anenhanced adhesion strength between the surface of the current collector3 and the first active material layer 2, it is preferable to use a foilhaving a large surface roughness (rugged foil). Moreover, by using asthe current collector 3 a rugged foil with a large surface roughness,bumps/dents corresponding to the surface of the current collector 3 arealso formed on the surface of the first active material layer 2. As aresult, through the aforementioned oblique vapor deposition, the activematerial particles 4 can be grown with a higher priority on the bumps ofthe first active material layer 2, which can advantageously ensureintervals between adjoining active material particles 4.

The surface roughness Ra of the current collector 3 is preferably 0.3 μmor more. As used herein, the “surface roughness Ra” refers to“arithmetic mean roughness Ra” as defined under the Japanese IndustrialStandards (JISB 0601-1994), and can be measured by using a surfaceroughness measurement system or the like, for example. When the surfaceroughness Ra is 0.3 μm or more, sufficient voids can be formed betweenadjoining active material particles 4 with an increased certainty. Onthe other hand, if the surface roughness Ra is too large, the currentcollector 3 will become thick, and therefore the surface roughness Ra ispreferably 5.0 μm or less. Furthermore, when the surface roughness Ra ofthe current collector 3 is within the aforementioned range (no less than0.3 μm and no more than 5.0 μm), sufficient adhesion force between thecurrent collector 3 and the first active material layer 2 can beensured, whereby peeling of the first active material layer 2 can beprevented.

The current collector 3 according to the present embodiment may bestructured so that a regular pattern of bumps/dents is formed on thesurface of the metal foil as described above. For example, grooves maybe formed on the surface of the current collector 3 to partition thesurface of the current collector 3 into a plurality of growth regionsfor growing active material particles 4. Preferably, the growth regionsare placed in a regular arrangement. With this structure, it is possibleto control the regions of the surface of the current collector 3 inwhich to grow the active material particles 4, whereby spaces for theactive material particles 4 to expand can be secured in the intervalsbetween adjoining active material particles 4 with an increasedcertainty.

FIGS. 3( a) and (b) are a cross-sectional view and a plan view,respectively, showing an exemplary current collector 3 having groovesformed on its surface. In the illustrated example, the surface of thecurrent collector 3 has a plurality of growth regions 7 partitioned byrectangular grooves 8. The depth of the grooves 8 is 10 μm, for example.Each growth region 7 has a diamond shape as viewed from the normaldirection of the surface of the current collector 3 (length of thediagonal: 10 μm×30 μm), and are placed in a regular arrangement so thatthe shortest distance P1 between the centers of adjoining growth regions7 is 30 μm. The arraying pitch P2 along the longer diagonal of thediamond shape is 60 μm. The surface of the growth region 7 may be flat,but preferably includes bumps/dents and has a surface roughness Ra of noless than 0.3 μm and no more than 5.0 μm, similarly to the surfaceroughness of the current collector 3 shown in FIG. 1.

The current collector 3 as shown in FIG. 3 may be formed by providing apredetermined pattern of grooves 8 on the surface of a metal foil (Cufoil) by using cutting technique, for example. Alternatively, aplurality of elevations may be formed on the surface of a metal foil byplating technique or transfer technique, and upper faces thereof may beutilized as growth regions 7.

When such a regular pattern of bumps/dents is formed on the surface ofthe current collector 3, the surface of the first active material layer2 will also have a corresponding pattern. When active material particles4 are formed on such a first active material layer 2 by oblique vacuumdeposition technique, the active material particles 4 will grow in therespective growth regions 7 via the first active material layer 2.Therefore, by appropriately selecting the pattern, size, arraying pitch,etc., of the growth regions 7 and grooves 8, it becomes possible toeffectively control the intervals between the active material particles4 and the film density.

The active material particles 4 in the present embodiment are notlimited to columnar particles which are tilted in one direction as shownin FIG. 1. For example, the active material particles 4 may grow alongthe normal direction D of the surface of the current collector 3. Suchactive material particles 4 can be obtained in the following manner.First, a photoresist is applied onto the current collector 3 having thefirst active material layer 2 formed thereon, and thereafter a maskingexposure treatment or the like is performed. Next, the non-exposedportions are removed by being dissolved through an etching treatment,whereby a resist having a pattern that corresponds to the voids betweenactive material particles 4 is formed on the current collector 3 havingthe first active material layer 2 formed thereon. Then, in the portionsof the surface of the current collector 3 (having the first activematerial layer 2 formed thereon) that are not covered with the resist,an active material is deposited by electroplating technique, vacuumdeposition technique, or the like, thus obtaining active materialparticles 4. Thereafter, the resist is removed.

Moreover, each active material particle 4 may have a plurality ofportions with different growth directions S. FIG. 4 is a schematiccross-sectional view illustrating active material particles 4 eachhaving a plurality of portions with different growth directions S. Inthis example, a current collector 3 having the structure which has beendescribed with reference to FIG. 3 is used. In other words, the surfaceof the current collector 3 is partitioned into a plurality of growthregions 7 by grooves 8. The active material particles 4 are formed inthe respective growth regions 7 of the current collector 3, and eachhave seven portions p1 to p7 with different growth directions S. Suchactive material particles 4 can be obtained by performing a plurality ofsteps of vapor deposition while changing the tilting angle. Such astructure can be confirmed by performing a chemical etching for apolished cross section which is perpendicular to the surface of thecurrent collector 3 of the electrode 6, and observing the sampleobtained.

Note that the second active material layer 5 only needs to containactive material particles 4, and may also contain active material layersor bodies of active material other than the active material particles.For example, in FIG. 1 and FIG. 4, layers having the same chemicalcomposition as that of the active material particles 4 may be providedon portions of the surface of the first active material layer 2 that arenot in contact with the active material particles 4. However, in view ofdeformation of the electrodes 6 due to expansions and contractions ofthe active material in the portions where such layers are provided, itis preferable that those layers have a thickness of 0.5 μm or less.

Hereinafter, with reference to the drawings, a method of producing theelectrode according to the present embodiment will be described, bytaking as an example a method of producing the electrode 6 shown in FIG.1.

First, the first active material layer 2 is formed on the currentcollector 3. Formation of the first active material layer 2 can beperformed by, using the vapor deposition apparatus as shown in FIG. 5,supplying silicon and oxygen onto the surface of the current collector3, and vapor-depositing a silicon oxide that has the desired chemicalcomposition.

The vapor deposition apparatus 100 shown in FIG. 5 includes a vacuumchamber 10 and an vacuum pump 25 for evacuating the vacuum chamber.Insider the vacuum chamber 10 are comprised: a supply roll 11 forretaining the current collector 3 before being subjected to vapordeposition; a take-up roll 12 for retaining the current collector 30after the first active material layer 2 has been formed; a substratecooling roll 14 placed between the rolls 11 and 12; an oxygen nozzle 17for emitting an oxygen gas; a Si raw material 22 as an evaporationsource for supplying silicon; an evaporation pot 23 for retaining the Siraw material 22; and an electron beam radiation system 24 forevaporating the Si raw material 22. Via tubing, the nozzle 17 isconnected to an oxygen flow rate controller 20 and an oxygen cylinder(not shown). A masking plate 15 is also provided inside the vacuumchamber 10 so that, through a gap in the masking plate 15 and from agenerally normal direction of the current collector 3, the oxygenemitted from the oxygen nozzle 17 and the Si atoms evaporated from theSi raw material 22 are supplied onto the surface of the currentcollector 3 on the substrate cooling roll 14. A region 26 defined by thegap in the masking plate 15, in which oxygen and Si atoms are suppliedonto the surface of the current collector 3, is referred to as a “firstactive material layer formation zone”.

A method for forming the first active material layer 2 by using thevapor deposition apparatus 100 as such will be specifically describedbelow.

A foil-like current collector 3 is placed on the supply roll 11, andwhile evacuating the vacuum chamber 10 with the vacuum pump 25, thefoil-like current collector 3 is moved from the supply roll 11 viapulleys 13 and allowed to travel along the substrate cooling roll 14. Inthe meantime, the evaporation pot 23 containing the Si raw material 22is irradiated with electrons from the electron beam radiation system 24,whereby Si is melted and evaporated. The evaporation amount of Si iscontrolled by using a film thickness monitor 19. At the same time,oxygen is introduced from the oxygen nozzle 17 into the vacuum chamber10. The flow rate of oxygen is controlled by the oxygen flow ratecontroller 20. Si and oxygen pass through the gap in the masking plate15, and, at the first active material layer formation zone 26, aresupplied onto the surface of the current collector 3 which is travelingon the substrate cooling roll 14. In the first active material layerformation zone 26, Si atoms are incident to and deposited on thetraveling current collector 3 in a generally normal direction, wherebythe first active material layer 2 is formed. The current collector 30having the first active material layer 2 formed thereon (whichhereinafter may also be simply referred to as the current collector 30)is wound up by the take-up roll 12.

Next, on the surface of the current collector 30 having the first activematerial layer 2 formed thereon, the second active material layer 5composed of a plurality of active material particles 4 is formed. Thus,the electrode 6 is obtained. Formation of the active material particles4 can be performed by, using a vapor deposition apparatus 200 as shownin FIG. 6, supplying silicon and oxygen onto the surface of the currentcollector 30, and vapor-depositing a silicon oxide having the desiredchemical composition. When performing such a vapor deposition, it isdesirable that the surface of the current collector 3 before forming thefirst active material layer 2 is roughened.

The vapor deposition apparatus 200 shown in FIG. 6 differs from thevapor deposition apparatus 100 shown in FIG. 5 in that the substratecooling roll 14, the Si raw material 22, the masking plate 15, theoxygen nozzle 17, and the like are placed in such a manner that oxygenand Si atoms are supplied onto the surface of the current collector 30from a direction which is tilted with respect to the normal direction ofthe current collector 30. For simplicity, constituent elements which aresimilar to those of the vapor deposition apparatus 100 shown in FIG. 5are denoted by like reference numerals, and descriptions thereof areomitted.

Note that, in the vapor deposition apparatus 200, a region 27 which isdefined by a gap in the masking plate 15, in which oxygen and Si atomsare supplied onto the current collector 30 from a direction that istilted with respect to the normal direction of the current collector 30,is referred to as an “active material particle-forming zone”.

A method for forming the second active material layer 5 by using thevapor deposition apparatus 200 as such will be specifically describedbelow.

The current collector 30 is placed on the supply roll 11, and whileevacuating the vacuum chamber 10 with the vacuum pump 25, the currentcollector 30 is moved from the supply roll 11 via the pulleys 13 andallowed to travel along the substrate cooling roll 14. In the meantime,Si raw material is melted and evaporated from the evaporation pot 23containing the Si raw material 22. At the same time, oxygen isintroduced from the oxygen nozzle 17 into the vacuum chamber 10. Theflow rate of oxygen is controlled by the oxygen flow rate controller 20.Si vapor and oxygen pass through the gap in the masking plate 15, and,at the active material particle-forming zone 27, are supplied onto thesurface of the current collector 3 which is traveling on the substratecooling roll 14. In the active material particle-forming zone 27, Siatoms are incident to and deposited on the traveling current collector30 from a direction which constitutes an angle θ (0°<θ<90°) with respectto the normal direction thereof. As a result, a plurality of activematerial particles 4 grow in a direction which is tilted with respect tothe normal direction of the current collector 3, and the second activematerial layer 5 composed of these active material particles 4 isformed. Thereafter, the current collector having the second activematerial layer 5 formed thereon is wound up by the take-up roll 12.Thus, the electrode 6 is obtained.

Note that, by only supplying Si vapor onto the surface of the currentcollector 3, active material particles 4 which are substantiallycomposed of Si alone (x=0 in SiO_(x)) may be formed. In this case,supply of oxygen from the oxygen nozzle 17 can be stopped by using theoxygen flow rate controller 20.

The thickness and composition (oxygen concentration) of the first activematerial layer 2 obtained by the aforementioned method can be controlledbased on, respectively, the time required for the current collector 3 topass through the first active material layer formation zone 26 and theoxygen flow rate from the oxygen nozzle 17. Similarly, the thickness andcomposition (oxygen concentration) of the active material particles 4can be controlled based on, respectively, the time required for thecurrent collector 30 to pass through the active materialparticle-forming zone 27 and the oxygen flow rate from the oxygen nozzle17. Since the substrate cooling roll 14 shown in FIG. 5 and FIG. 6 has aconstant rotation speed, the deposition rate of Si atoms at the firstactive material layer formation zone 26 and the active materialparticle-forming zone 27 can be calculated from the generally knowncos-rule. Moreover, based on the position of the masking plate 15, thetime required for the current collector 3, 30 to pass through the firstactive material layer formation zone 26 or the active materialparticle-forming zone 27 can be set as appropriate.

As the evaporation pot 23, a carbon pot (which is commonly used formelting Si) is used. The silicon purity of the Si raw material 22 shouldbe as high as possible, and is 99.99% or more, for example. The methodfor heating the Si raw material 22 may be an induction heatingtechnique, a resistance heating technique, a heating technique based onelectron beam irradiation, or the like.

The oxygen amount in the first active material layer formation zone 26and in the active material particle-forming zone 27 can be appropriatelyselected depending on manufacturing conditions such as: the amount ofoxygen to be introduced from the oxygen nozzle 17, the shape of thevacuum chamber 10, the evacuation ability of the vacuum pump 25, theevaporation rate of the Si raw material 22, and the width of the firstactive material layer formation zone 26 and the active materialparticle-forming zone 27 along the travel direction of the currentcollector 3, 30.

Methods for forming the first active material layer 2 and the secondactive material layer 5 are not particularly limited to the abovemethods, but it is preferable to use a dry process such as a vapordeposition technique, a sputtering technique, or a CVD technique. Inparticular, a vapor deposition technique is a method which providesexcellent productivity. Use of a vapor technique is advantageous becauseit allows the first active material layer 2 and second active materiallayer 5 to be formed on the moving current collector 3 in a continuousmanner and in large quantity.

In the methods described above with reference to FIG. 5 and FIG. 6, thecurrent collector 30 after having formed the first active material layer2 is temporarily wound up, and thereafter is again attached to the vapordeposition apparatus for forming the second active material layer 5.However, formation of the first active material layer 2 and the secondactive material layer 5 may be carried out within the same vacuumchamber. For example, two substrate cooling rolls may be provided in thevacuum chamber, and after the first active material layer 2 is formed onthe surface of the current collector 3 upon one substrate cooling roll,the second active material layer 5 may be formed upon the othersubstrate cooling roll, without winding up the current collector 3.Alternatively, the first active material layer 2 and the second activematerial layer 5 may be successively formed on the same substratecooling roll.

In the case where the first active material layer 2 and the secondactive material layer 5 are to be formed in the same vacuum chamber, thefirst active material layer formation zone 26 for forming the firstactive material layer 2 and the active material particle-forming zone 27for forming the active material particles 4 may be placed in closeproximity, and the active material particles 4 may be grown immediatelyafter formation of the first active material layer 2, whereby anelectrode 6 whose oxygen concentration gradually changes at the bondingsites between the first active material layer 2 and the active materialparticles 4 can be easily produced. Moreover, forming the first activematerial layer 2 and the active material particles 4 (i.e., the secondactive material layer 5) by using the same evaporation source (e.g., Siraw material) can advantageously simplify the production equipment.

In the methods described above with reference to FIG. 5 and FIG. 6, thefirst active material layer 2 and the second active material layer 5 areformed for the traveling current collector 3, 30 upon the roller(substrate cooling roll) 14. However, an endless belt may be usedinstead of a roller, and the first active material layer 2 and thesecond active material layer 5 may be formed for the current collector3, 30 which is traveling upon a linearly-extending portion of theendless belt. Alternatively, the first active material layer 2 and thesecond active material layer 5 may be formed while fixing the currentcollector 3, 30 within the vacuum chamber.

Next, with reference to the drawings, an exemplary construction of alithium-ion secondary battery which is obtained by applying theelectrode 6 of the present embodiment will be described.

FIG. 7 is a schematic cross-sectional view illustrating a coin-typelithium-ion secondary battery in which the electrode of the presentembodiment is used. The lithium-ion secondary battery 50 includes anelectrode 52, a positive electrode 54 containing a positive-electrodeactive material such as LiCoO₂, LiNiO₂, or LiMn₂O₄, and a separator 58provided between the electrode 52 and the positive electrode 54 andbeing composed of a microporous film or the like. Together with anelectrolyte having lithium-ion conductivity, the electrode 52, thepositive electrode 54, and the separator 58 are accommodated within thecase 64 by a sealing plate 62 having a gasket 60. A stainless steelspacer 56 for filling up the space (shortage of intra-case height) inthe case 64 is also placed inside the case 64.

The surface of the electrode 52 facing the positive electrode 54 has theconstruction described above with reference to FIG. 1. Moreover, as anelectrolyte having lithium-ion conductivity, an electrolyte of agenerally known composition, e.g., an electrolyte obtained by dissolvinglithium hexafluorophosphate or the like in a cyclic carbonate such asethylene carbonate or propylene carbonate may be used, for example.

Note that the construction of a lithium-ion secondary battery having theelectrode 6 of the present embodiment is not limited to the constructionshown in FIG. 7. Other than a coin-type lithium-ion secondary battery,the electrode 6 of the present embodiment is also applicable tolithium-ion secondary batteries of various shapes, e.g., cylindrical,flat, or prismatic. Moreover, the manner of sealing the lithium-ionsecondary battery and the materials of the respective elements composingthe battery are not particularly limited either. Furthermore, it is alsoapplicable to a non-aqueous electrolyte secondary battery other than alithium-ion secondary battery.

Second Embodiment

Hereinafter, a method of producing an electrode according to a secondembodiment of the present invention will be described. The method of thepresent embodiment differs from the methods which have been describedwith reference to FIG. 5 and FIG. 6 in that the first active materiallayer 2 and the second active material layer 5 are successively formedby using the same evaporation source, within the same vacuum chamber.

FIG. 8 is a diagram for explaining the production method according tothe present embodiment. For simplicity, FIG. 8, constituent elementswhich are similar to those of the vapor deposition apparatus 100 and 200shown in FIG. 6 are denoted by like reference numerals, and descriptionsthereof are omitted.

In the present embodiment, a vacuum deposition apparatus 300 shown inFIG. 8 is used to form the second active material layer 5, which iscomposed of the first active material layer 2 and the active materialparticles 4, on the surface of the foil-like current collector 3.

In the vapor deposition apparatus 300 shown in FIG. 8, upper and lowersubstrate cooling rolls 14A and 14B, a Si raw material 22, an uppermasking plate 15, a lower masking plate 16, first and second oxygennozzles 17 and 18, and the like are placed so as to create: an activematerial particle-forming zone 27, in which oxygen and Si atoms aresupplied to the current collector 3 from a direction that is tilted withrespect to the normal direction of the current collector 3; and a firstactive material layer formation zone 26, which is provided immediatelybefore the active material particle-forming zone 27 and in which theoxygen concentration is increased than in the active materialparticle-forming zone 27.

A method for forming the first active material layer 2 and the secondactive material layer 5 by using the vapor deposition apparatus 300 assuch will be specifically described.

The foil-like current collector 3 is placed on the supply roll 11, andwhile evacuating the vacuum chamber 10 with an vacuum pump 25, thecurrent collector 3 is moved from the supply roll 11 via the pulleys 13,and allowed to travel along the lower and upper substrate cooling rolls14A and 14B. In the meantime, an evaporation pot 23 containing the Siraw material 22 is irradiated with electrons from an electron beamradiation system 24, whereby the Si raw material is melted andevaporated. The evaporation amount of Si is controlled by a filmthickness monitor 19. At the same time, oxygen is introduced into thevacuum chamber 10 from the first oxygen nozzle 17 and the second oxygennozzle 18. The amounts of oxygen emitted from the nozzles 17 and 18 arecontrolled by a first oxygen flow rate controller 20 and a second oxygenflow rate controller 21, respectively. As shown in the figure, withinthe emission range of Si atoms, a region that is defined by a gapbetween the upper masking plate 15 and the lower masking plate 16 servesas the “active material particle-forming zone” 27. On the other hand, aregion that lies outside of the emission range of the Si atoms but islocated between the lower masking plate 16 and the lower substratecooling roll 14A, in which Si atoms are indirectly supplied to thecurrent collector 3 through collision with gaseous molecules, etc.,serves as the “first active material layer formation zone” 26.

The current collector 3 traveling from the supply roll 11 first passesthrough the first active material layer formation zone 26. Here, at thesurface of the current collector 3, the Si atoms which are indirectlysupplied through collision with gaseous molecules or the like react withthe oxygen which is supplied from the first oxygen nozzle 17 and thesecond oxygen nozzle 18 and which has arrived in between the lowermasking plate 16 and the lower substrate cooling roll 14, whereby thefirst active material layer 2 is formed. Therefore, unlike in the activematerial particle-forming zone 27, there is no directionality as to thepositions at which Si atoms are vapor-deposited, and no shadowing effectas described later occurs. As a result, the first active material layer2 is generally uniformly formed across the entire surface of the currentcollector 3. The vapor deposition rate of the Si atoms is controlled byadjusting the distance (interspace) between the lower masking plate 16and the current collector 3. The thickness of the first active materiallayer 2 is determined by the evaporation rate of Si, the distancebetween the melting surface of the evaporation material and the lowermasking plate 16, the interspace between the lower masking plate 16 andthe current collector 3, the transportation speed of the currentcollector 3, and the like. For example, assuming that the Si evaporationrate from the evaporation pot 23 is 0.04 g/sec, the vertical distancefrom the Si melting surface in the evaporation pot 23 to the lowermasking plate 16 is 435 mm, the interspace between the lower maskingplate 16 and the current collector 3 is 3 mm, and the transportationspeed of the current collector 3 is 0.4 cm/min, the first activematerial layer 2 has a thickness of about 50 nm.

After the first active material layer 2 is formed, the current collector3 moves to the active material particle-forming zone 27. In the activematerial particle-forming zone 27, Si vapor and oxygen are supplied ontothe surface of the current collector 3, whereby the second activematerial layer 5 composed of a plurality of active material particles 4is obtained on the first active material layer 2. Here, Si atoms areincident on the traveling current collector 3 from a direction which isat an angle θ (0°<θ<90°) with respect to the normal direction of thecurrent collector 3, so that the active material particles 4 grow in adirection which is tilted from the normal direction of the currentcollector 3. Since the Si atoms are incident to the surface of thecurrent collector 3 from the specific direction, they are likely to bevapor-deposited upon the bumps (bumps 3 a shown in FIG. 1) on thesurface of the current collector 3, thus resulting in the silicon oxidegrowing in columnar shapes only on the bumps. On the other hand, in anyportion of the surface of the current collector 3 that is shaded by acolumnar growth of silicon oxide, the Si atoms are not incident and nosilicon oxide is vapor-deposited (shadowing effect).

Thereafter, the current collector having the second active materiallayer 5 formed thereon is wound up by the take-up roll 12. Thus, theelectrode 6 is obtained.

With the above-described method, the active material particle-formingzone 27 and the first active material layer formation zone having ahigher oxygen concentration than in the active material particle-formingzone 27 can be placed in close proximity, within the same vacuumchamber. This makes it possible to successively form the first activematerial layer 2 and the second active material layer 5 by using thesame evaporation source (Si raw material) 22. Therefore, the number ofproduction steps can be reduced, and the production equipment can besimplified. Moreover, with the above-described method, while allowingthe current collector 3 to travel, the step of forming the first activematerial layer 2 and the step of forming the second active materiallayer 5 are simultaneously performed, whereby the time required formanufacture can also be reduced.

EXAMPLES

Examples of the electrode for a lithium-ion secondary battery accordingto the present invention will be described. Herein, electrodes ofExamples 1 and 2 were produced by using the methods described inEmbodiments 1 and 2, respectively. For comparison, an electrode lackingthe active material layer was produced as Comparative Example.

Example 1

With the methods described above with reference to FIG. 5 and FIG. 6, anelectrode for a lithium-ion secondary battery of Example 1 was produced.The specific production method will be described.

First, by using the vapor deposition apparatus 100 shown in FIG. 5, thefirst active material layer 2 was formed on the surface of the currentcollector 3. A Cu foil (thickness: 40 μm) having a surface roughness Raof 2.0 μm was used as the current collector 3. Prior to vapordeposition, vacuum evacuation was performed by using the vacuum pump 25until the interior of the vacuum chamber 10 reached 3×10⁻³ Pa.Thereafter, in the vacuum chamber 10, the current collector 3 was movedfrom the supply roll 11 onto the substrate cooling roll 14 via thepulleys 13, and the current collector 3 was allowed to travel along thesubstrate cooling roll 14 at a speed of 4 m/min (speed of substratetravel). In the meantime, the evaporation pot 23 containing 200 g of Siraw material 22 was irradiated with electrons from the electron beamradiation system 24, which were accelerated at −10 kV, thus melting andevaporating the Si raw material. The evaporation amount of Si was 0.04g/sec. On the other hand, oxygen was introduced into the vacuum chamber10 from the oxygen nozzle 17. At this time, the oxygen nozzle 17 wasplaced so that the tip of the oxygen nozzle 17 was located at a heightof 200 mm from the evaporation pot 23, and that the direction of oxygenemission was aimed toward the current collector 3 traveling along thesubstrate cooling roll 14 through the gap in the masking plate 5.Moreover, the flow rate of oxygen from the oxygen nozzle 17 wascontrolled to 1200 sccm by using the oxygen flow rate controller 20.Upon oxygen introduction, the vacuum chamber 10 had a degree of vacuumof 4.5×10⁻² Pa (degree of vacuum during vapor deposition).

In the present Example, the positions of the evaporation pot 23 and thegap in the masking plate 15, the width of the gap in the masking plate15, and the like were set so that Si atoms would be incident to thetraveling current collector 3 from a direction at an angle θ of no lessthan −13° and no more than +13° with respect to the normal of thecurrent collector 3. The distance between the evaporation pot 23 and thecurrent collector 3 (distance along a direction in which the above angleθ was 0°) was 400 mm.

In this manner, Si atoms and oxygen were supplied to allow silicon oxideto be vapor-deposited on the surface of the current collector 3, thusforming the first active material layer 2. The resultant first activematerial layer 2 had a thickness of 90 nm. Thereafter, the currentcollector 30 having the first active material layer 2 formed thereon waswound up onto the take-up roll 12.

Next, by using the vapor deposition apparatus 200 shown in FIG. 6, theactive material particles 4 was grown on the first active material layer2, thus forming the second active material layer 5. Prior to vapordeposition, the vacuum evacuation was performed by using the vacuum pump25 until the interior of the vacuum chamber 10 reached 3×10⁻³ Pa.Thereafter, in the vacuum chamber 10, the current collector 30 was movedfrom the supply roll 11 onto the substrate cooling roll 14 via thepulleys 13, and the current collector 30 was allowed to travel along thesubstrate cooling roll 14 at a speed of 1.0 cm/min (speed of substratetravel). In the meantime, the evaporation pot 23 containing 200 g of Siraw material 22 was irradiated with electrons from the electron beamradiation system 24, which were accelerated at −10 kV, thus melting andevaporating the Si raw material. The evaporation amount of Si was 0.04g/sec. On the other hand, oxygen was introduced into the vacuum chamber10 from the oxygen nozzle 17. At this time, the oxygen nozzle 17 wasplaced so that the tip of the oxygen nozzle 17 was located at a heightof 200 mm from the evaporation pot 23, and that the direction of oxygenemission was aimed toward the current collector 3 traveling along thesubstrate cooling roll 14 through the gap in the masking plate 15.Moreover, the flow rate of oxygen from the oxygen nozzle 17 wascontrolled to 700 sccm by using the oxygen flow rate controller 20. Uponoxygen introduction, the vacuum chamber 10 had a degree of vacuum(degree of vacuum during vapor deposition) of 2.5×10⁻² Pa.

In the present Example, the positions of the evaporation pot 23 and thegap in the masking plate 15, the width of the gap in the masking plate15, and the like were set so that Si atoms would be incident to thetraveling current collector 30 from a direction at an angle θ of no lessthan 55° and no more than +82° with respect to the normal of the currentcollector 30. The distance between the evaporation pot 23 and thecurrent collector 30 (distance along a direction in which the aboveangle θ was 55°) was 435 mm.

In this manner, Si atoms and oxygen were supplied to allow silicon oxideto be vapor-deposited on the surface of the current collector 30, thusforming the second active material layer 5 containing the activematerial particles 4. Thus, the electrode of Example 1 was obtained. Thesecond active material layer 5 had a thickness of 20 μm.

Note that the thicknesses of the first active material layer 2 and thesecond active material layer 5 were determined by a measurement methodwhich is described later; the same is also true of the following Exampleand Comparative Example.

Example 2

With the methods described above with reference to FIG. 8, an electrodefor a lithium-ion secondary battery of Example 2 was produced.

In the present Example, the positions of the evaporation pot 23 and thegap between the masking plates 15 and 16 were set so that Si atoms wouldbe incident to the traveling current collector 3 from a direction at anangle θ of no less than 65° and no more than 72° with respect to thenormal of the current collector 3, in the active materialparticle-forming zone 27 of the vapor deposition apparatus 300 shown inFIG. 8. The distance between the evaporation pot 23 and the currentcollector 3 (distance along a direction in which the above angle θ was65°) was 550 mm. Furthermore, a gap of 4 mm was provided between thelower substrate 16 and the current collector 3, thus creating the firstactive material layer formation zone 26 between the lower masking plate16 and the lower substrate cooling roll 14A.

A specific production method will be described below.

After placing the current collector 3 on the supply roll 11, prior tovapor deposition, vacuum evacuation was performed by using the vacuumpump 25 until the interior of the vacuum chamber 10 reached 3×10⁻³ Pa.Thereafter, in the vacuum chamber 10, the current collector 3 was movedfrom the supply roll 11 onto the upper and lower substrate cooling rolls14A and 14B via the pulleys 13, and the current collector 3 was allowedto travel along the substrate cooling rolls 14A and 14B at a speed of1.4 cm/min (speed of substrate travel). A Cu foil (thickness: 40 μm)having a surface roughness Ra of 2.0 μm was used as the currentcollector 3. In the meantime, the evaporation pot 23 containing 200 g ofSi raw material 22 was irradiated with electrons from the electron beamradiation system 24, which were accelerated at −10 kV, thus melting andevaporating the Si raw material. The evaporation amount of Si was 0.06g/sec.

At the same time, oxygen was introduced from the first oxygen nozzle 17and the second oxygen nozzle 18 into the vacuum chamber 10. The oxygenflow rate from the first oxygen nozzle 17 was controlled to 100 sccm byusing the first oxygen flow rate controller 20, and the flow rate ofoxygen from the second oxygen nozzle 18 was controlled to 100 sccm byusing the second oxygen flow rate controller 21. The direction of oxygenemission from the first oxygen nozzle 17 was generally perpendicular tothe current collector 3 traveling along the substrate cooling roll 14through the gap in the masking plate 5, and the direction of oxygenemission from the second oxygen nozzle 18 was generally parallel to thecurrent collector 3 traveling along the substrate cooling roll 14 viathe gap in the masking plate 5. Moreover, the tips of the first andsecond oxygen nozzles 17 and 18 were placed at a height of 350 mm fromthe evaporation pot 23.

The current collector 3 from the supply roll 11 first passed through thefirst active material layer formation zone 26. Here, at the surface ofthe current collector 3, the Si atoms having been indirectly suppliedthrough collision or the like reacted with the oxygen having beensupplied from the first oxygen nozzle 17 and the second oxygen nozzle 18to arrive in between the lower masking plate 16 and the lower substratecooling roll 14, whereby the first active material layer 2 was formed.The first active material layer 2 of the present Example was generallyuniformly formed across the entire surface of the current collector 3.

Next, the current collector 3 having the first active material layer 2formed thereon was moved into the active material particle-forming zone27. In the active material particle-forming zone 27, the Si atomsemitted from the Si raw material 22 reacted with the oxygen suppliedfrom the first oxygen nozzle 17 and the second oxygen nozzle 18, wherebythe second active material layer 5 composed of a plurality of activematerial particles 4 was formed on the first active material layer 2. Inthe present Example, as described above, Si atoms were incident to thesurface of the current collector 3 from a direction at an angle θ(65°≦θ≦72°) with respect to the normal direction of the currentcollector 3, and therefore the active material particles 4 grew in adirection which was tilted with respect to the normal direction of thecurrent collector 3.

In the electrode of Example 2 obtained by the above-described methods,the first active material layer 2 had a thickness of 50 nm, and thesecond active material layer 5 had a thickness of 20 μm. Moreover, theangle between the growth direction of the active material particles 4and the normal direction of the current collector 3 was about 38°.

Comparative Example

An electrode of Comparative Example was produced by forming a secondactive material layer directly on the surface of a current collector,without forming a first active material layer. Thus, the construction ofthe electrode of Comparative Example is similar to the constructiondescribed above with reference to FIG. 2. Similarly to Examples 1 and 2,a Cu foil (thickness: 40 μm) having a surface roughness Ra of 2.0 μm wasused as the current collector. Formation of the second active materiallayer was performed by using the vapor deposition apparatus 200 shown inFIG. 6, according to a method and conditions similar to the method andconditions for forming the second active material layer 5 of Example 1.The resultant second active material layer had a thickness of 20 μm.

(Analyses and Evaluations)

Analyses/evaluations of the electrodes of the Examples and ComparativeExample obtained with the above-described methods were made. The methodand results thereof will be described. Herein, the chemical compositionsand thicknesses of the first active material layers and the activematerial particles in the electrodes of the Examples and ComparativeExample were measured. Furthermore, batteries were produced using theseelectrodes, and measurements and evaluations of their cyclecharacteristics were made.

1. Chemical Composition

The chemical compositions of the active material particles 4 of Example1 and Example 2 were measured in the following manner.

First, after forming the active material particles 4, samples for activematerial particle measurement sized 1 cm×1 cm were cut out, out of thewound-up current collector 3, from a portion on which the activematerial particles 4 were initially formed during the vapor depositionprocess (start of film formation) and from a portion on which the activematerial particles 4 were finally formed (end of film formation).

Next, the Si amounts and the oxygen amounts in such measurement sampleswere measured with an ICP Atomic Emission Spectrometer and by combustionanalysis technique, respectively, and based on average values of the Siamounts and oxygen amounts obtained, a molar ratio (mole fraction ofoxygen) “x” of the oxygen amount with respect to the Si amount in theactive material particles 4 was calculated.

Note that each sample for active material particle measurement includesthe first active material layer 2 in addition to the active materialparticles 4, and therefore the Si amount and oxygen amount obtainedthrough the measurements would also include the Si amount and oxygenamount within the first active material layer 2. However, since thethickness of the first active material layer 2 is much smaller ( 1/222)than the thickness of the active material particles 4, the influences ofthe Si amount and oxygen amount in the first active material layer 2exerted on the oxygen concentration in the active material particles 4are very small. Accordingly, in the present Example, the mole fractionof oxygen in each aforementioned sample for active material particlemeasurement was used as the mole fraction of oxygen “x” of the activematerial particles 4.

Moreover, the chemical composition of the first active material layers 2of Example 1 and Example 2 were measured in the following manner.

In Example 1, after forming the first active material layer 2, samplesfor first active material layer measurement sized 1 cm×1 cm were cutout, out of the current collector 3 before formation of the secondactive material layer 5, from a portion on which the first activematerial layer 2 was initially formed during the vapor depositionprocess for forming the first active material layer 2 (start of filmformation) and from a portion on which the first active material layer 2was finally formed (end of film formation). Thereafter, as in theaforementioned measurement method of the chemical composition of theactive material particles 4, the Si amount was measured with ICP AtomicEmission Spectrometry and the oxygen amount was measured by combustionanalysis technique in each measurement sample, and based on averagevalues of the Si amounts and oxygen amounts obtained, a molar ratio(mole fraction of oxygen) “y” of the oxygen amount with respect to theSi amount in the first active material layer 2 was calculated.

In Example 2, samples for first active material layer measurement sized1 cm×1 cm were cut out, out of the current collector 3 after formationof the first active material layer 2 and the second active materiallayer 5, from a portion on which the first active material layer 2 andthe second active material layer 5 were initially formed during thevapor deposition process (start of film formation) and from a portion onwhich the first active material layer 2 and the second active materiallayer 5 were finally formed (end of film formation). Then, each samplefor first active material layer measurement was subjected to Ar etchingto expose the first active material layer 2, and an Si_(2p) bindingenergy at the exposed surface was measured by using X-ray PhotoelectronSpectroscopy (XPS), whereby an average value “y” of the mole fraction ofoxygen in the first active material layer 2 was calculated.

Furthermore, with respect to the electrode of Comparative Example, too,the chemical composition of the active material particles was measuredwith a similar method to the above.

As a result of the aforementioned measurements, it was found that theactive material particles 4 of Example 1 had a chemical composition ofSiO_(0.8)(x=0.8), and that the active material particles 4 of Example 2had a chemical composition of SiO_(0.7)(x=0.7). The active materialparticles of Comparative Example had the same chemical composition asthat of Example 1, i.e., SiO_(0.8)(x=0.8).

Moreover, the chemical composition of the first active material layer 2was SiO_(1.2)(y=1.2) in both Example 1 and Example 2.

2. Thicknesses of the First Active Material Layer and the Second ActiveMaterial Layer

The thicknesses of the first active material layer 2 and the secondactive material layer 5 of each Example were determined by using aMarcus radio frequency glow discharge spectrometer from HORIBA on thesamples for first active material layer measurement and samples foractive material particle measurement of the respective Examplesdescribed above. With a similar method, the thickness of the secondactive material layer of Comparative Example was also calculated.

3. Production of Coin Batteries and Cycle Characteristics Evaluations ofthe Coin Batteries

First, coin batteries were produced by using the electrodes of theExamples and Comparative Example, which were named Cell Samples a, b,and c for evaluation. These Cell Samples had a similar construction tothe construction shown in FIG. 7.

Hereinafter, with reference to FIG. 7, a production method for the CellSamples will be specifically described.

The electrode 6 of Example 1 was each cut out in a circular shape(diameter: 12.5 mm), thus forming an electrode 52 for a coin battery.The electrode 52 for a coin battery was placed so as to oppose ametal-lithium positive electrode (thickness: 300 μm, diameter: 15 mm) 54via a polyethylene separator (thickness 25 μm, diameter 17 mm) 56, andinserted in a “2016” size coin battery case (total height: 1.6 mm,diameter: 20 mm) 64.

Next, an electrolyte obtained by dissolving a solute of 1M LiPF₆ in a1:1 (volume ratio) mixed solvent of ethylene carbonate and diethylcarbonate was injected into the coin battery case 64. Moreover, in orderto fill up the space (shortage of intra-case height) in the case 64, thestainless steel spacer 56 was inserted. Thereafter, a sealing plate 62having a polypropylene gasket 60 around its periphery was placed on thebattery case 64, and the battery case 64 was crimped at the periphery,thus producing Cell Sample a having the electrode of Example 1.

Moreover, with a similar method to the above, Cell Samples b and chaving the electrodes of Example 2 and Comparative Example wereproduced.

Next, charge-discharge capacities of Cell Samples a, b, and c (astwo-electrode cells) were measured, and their cycle characteristics wereexamined. The method and results thereof will be described.

The charge-discharge capacity measurement of each Cell Sample was takenby subjecting the Cell Sample to repetitive charge-discharge cycles,each consisting of charging to 0V with a constant current of 1 mA anddischarging to 1.5V with a constant current of 1 mA (constant currentcharge-discharge method). The ambient temperature during the measurementwas room temperature (e.g., 25° C.).

The measurement results are shown in FIG. 9. FIG. 9 is a graph showingthe charge-discharge cycle characteristics of each Cell Sample, wherethe horizontal axis of the graph represents the number of cycles(times), and the vertical axis represents the discharge capacityretention (%).

From the results shown in FIG. 9, it has been found that Cell Samples aand b having the electrodes of Examples 1 and 2 maintain a 90% or morecapacity of the initial capacity after the lapse of 150 cycles, thushaving excellent charge-discharge cycle characteristics. On the otherhand, the capacity of Cell Sample c having the electrode of ComparativeExample begins to decrease after the lapse of 100 cycles, and lowersdown to 80% of the initial capacity after the lapse of 150 cycles. Thisis presumably because peeling of the active material particles occurreddue to expansions and contractions of the active material particlesassociated with charge and discharge. Therefore, it was confirmed thatthe charge-discharge cycle characteristics can be improved by providingthe first active material layer 2 having a smaller expansion coefficientthan that of the active material particles 4 between the active materialparticles 4 and the current collector 3.

Note that the electrode for a non-aqueous electrolyte secondary batteryaccording to the present invention is also applicable to an electrodefor lithium occlusion/release in an electrochemical capacitor. As aresult, an electrochemical capacitor having an energy density thatcannot be achieved with a conventional graphite electrode can beobtained.

INDUSTRIAL APPLICABILITY

The electrode for a non-aqueous electrolyte secondary battery accordingto the present invention is applicable to non-aqueous electrolytesecondary batteries such as various lithium-ion secondary batteries,e.g., coin-, cylindrical-, flat-, or cubic-type. These non-aqueouselectrolyte secondary batteries have excellent cycle characteristics,and therefore can be broadly used in: mobile information terminals suchas PCs, mobile phones, and PDAs; audio-visual devices such asvideorecorders, memory audio players; and so on.

1. An electrode for a non-aqueous electrolyte secondary battery,comprising: a current collector; a first active material layer formed onthe current collector; and a second active material layer provided onthe first active material layer, the second active material layerincluding a plurality of active material particles, wherein, theplurality of active material particles are mainly of a chemicalcomposition represented as SiOx(0≦x<1.2); the first active materiallayer is mainly of a chemical composition represented as SiOy(1.0≦y<2.0,y>x); and an area in which the first active material layer is in contactwith the plurality of active material particles is smaller than an areain which the current collector is in contact with the first activematerial layer.
 2. The electrode for a non-aqueous electrolyte secondarybattery of claim 1, wherein a growth direction of the plurality ofactive material particles is tilted with respect to a normal directionof the current collector.
 3. The electrode for a non-aqueous electrolytesecondary battery of claim 1, wherein the first active material layerand the active material particles go through gradual changes in chemicalcomposition at bonding sites between the first active material layer andthe plurality of active material particles.
 4. The electrode for anon-aqueous electrolyte secondary battery of claim 1, wherein a ratios2/s1 of the area s2 in which the first active material layer is incontact with the plurality of active material particles to an area s1 ofthe first active material layer is no less than 20% and no more than70%.
 5. The electrode for a non-aqueous electrolyte secondary battery ofclaim 1, wherein the first active material layer has a thickness greaterthan 2 nm and less than 100 nm.
 6. The electrode for a non-aqueouselectrolyte secondary battery of claim 1, wherein the current collectorhas a surface roughness Ra of no less than 0.3 μm and no more than 5 μm.7. The electrode for a non-aqueous electrolyte secondary battery ofclaim 1, wherein, the current collector includes: a groove formed on asurface; and a plurality of growth regions partitioned by the groove,wherein, each of the plurality of active material particles is formed ona corresponding growth region with the first active material layerinterposed therebetween.
 8. A non-aqueous electrolyte secondary batterycomprising: a positive electrode capable of occluding and releasinglithium ions; the electrode for a non-aqueous electrolyte secondarybattery of any of claim 1; a separator disposed between the positiveelectrode and the electrode for a non-aqueous electrolyte secondarybattery; and an electrolyte having lithium-ion conductivity.
 9. A methodof producing an electrode for a non-aqueous electrolyte secondarybattery, comprising: (A) a step of providing a sheet-like currentcollector; and (B) a step of forming, within a same chamber, a firstactive material layer and a second active material layer upon thecurrent collector by supplying silicon and oxygen onto a surface of thecurrent collector, wherein, step (B) includes: a step (b1) of forming,in a first region within the same chamber, forming a first activematerial layer of a chemical composition represented as SiOy on thesurface of the current collector; a step (b2) of, by using a roller,moving the current collector from the first region to a second regionwithin the same chamber; and a step (b3) of forming, in the secondregion, a plurality of active material particles upon the first activematerial layer, the active material particles being of a chemicalcomposition represented as SiOx(x<y), thus forming the second activematerial layer including the plurality of active material particles. 10.The method of producing an electrode for a non-aqueous electrolytesecondary battery of claim 9, wherein silicon is supplied by using asame evaporation source in steps (b1) and (b3).
 11. The method ofproducing an electrode for a non-aqueous electrolyte secondary batteryof claim 9, wherein steps (b1) and (b3) are simultaneously performed.12. The method of producing an electrode for a non-aqueous electrolytesecondary battery of claim 11, wherein step (b2) is simultaneouslyperformed with steps (b1) and (b3).
 13. The method of producing anelectrode for a non-aqueous electrolyte secondary battery of claim 9,wherein, the plurality of active material particles are of a chemicalcomposition represented as SiOx(0≦x<1.2); and the first active materiallayer is of a chemical composition represented as SiOy(1.0≦y≦2.0, y>x).